1. Field of the Invention
The present invention relates to compositions and methods for the manufacture and repair of composite structures that contain conductive layers and, more particularly, to methods using nanotubes for manufacturing and repairing fiber-reinforced composite structures that provide and maintain EM and lightning protection.
2. Description of the Background
Composite structures that contain conductive layers to impart electromagnetic (EM) and lightning strike protection to the structure are known in the art. These composites can function alone, or in conjunction with other structural parts, to protect an entire enclosure. Typically, these types of composites may be found in fiber-reinforced polymer matrix composite walls, decks, roofs of a building, ships, vehicles, aerospace crafts, electrical housings, etc. In addition to the mechanical benefits of using such composite structures, the fabrication of these structures provides an electrically conductive layer that protects the structure from damage from lightening strikes (see publication numbers MIL-HDBK-1004/6, 1988; AD-A 252 281, 1992; NRL/MR1465492-6986) and transmission of broadband EM radiation.
The protective conductive layer is installed during the fabrication of the structure and remains functional until damaged. Damage can result from a lightning strike, or mechanical impact or stress from deliberate enemy attack. Resulting damage can be exhibited as a crack or hole, or any separation that interrupts electrical conductivity in the area of the damage, which thereby allows the transmission of EM and reduces the conductive pathway for mitigation of lightening strike.
Known techniques used to repair the structure's electrical and mechanical properties typically involve cutting the damaged area from the structure, routing the hole to form steps at each layer of the composite, and filling the whole with new layers of fiber and resin. For an example of such a process, see Composite Repair, available from Fibre Glast Developments Corporation, Brookville, Ohio.
If the conductive layer is included in the repair patch, that patch must make electrical contact to the electrical layer existing in the damaged structure to provide the same layer of EM and lightening protection as the original structure. The electrically conductive layer in the structure typically includes conductive materials, such as metal-coated fibers of carbon, glass, or polymers, foils of metals, and screens of metals. Electromagnetic materials, and their properties, are known in the art. See Neelakanta, Perambur S. Handbook of Electromagnetic Materials, CRC Press, Inc., New York, 1995. Once the layers of the composite are damaged, however, making electrical contact with the metal coating on the fibers forming the protective layer is very difficult and often practically impossible such that the damaged structure is considered damaged beyond repair and an entirely new structure must be manufactured.
In traditional patch repair techniques, the two conductive layers have only incidental contact wherever the conductive fibers happened to make contact, even if some effort is made to commingle the fibers from the patch to those in the structure. Exposure of the structures conductive a fiber is very difficult and does not guarantee that contact between the repair fiber and the structure will occur during cure and final assemble of the other nonconductive (structural) layers of the patch.
The level of difficulty of the repair is greatly increased when such repairs must be conducted during armed conflicts, such as in the repair of a naval ship during battle. The damaged ship is both more vulnerable to attack and less capable of defending itself due to the loss of EM protection.